The present invention relates to substantially silanol-free silicone resin powder having a primary particle size in the range of 0.1-200 nm and an aggregate particle size of 10 nm to 200 microns. More particularly, the present invention relates to a method involving the pretreatment of an organic solvent dispersion of an organosiloxane hydrolyzate with an organosilicon nitrogen material, such as an organosilylamine, or organosilazane, and the spray drying of the resulting mixture to form a substantially silanol-free silicone resin powder useful for making heat curable organopolysiloxane compositions having improved processing characteristics.
A method for making silicone resin powder is shown by Wolfgruber et al, U.S. Pat. No. 4,935,484. An aqueous colloidal suspension of an organo siloxane copolymer is spray dried to produce a silicone resin powder having an average particle diameter of 10 to 150 nm. Although spray dried powders made by Wolfgruber et al can be utilized in a variety of applications as fillers, the powders are contaminated with the emulsifying agent which was was used in the preparation of the organosiloxane copolymer. As taught by Wolfgruber et al, silicone resin powder made by spray drying an aqueous silicone mixture can be used as a substitute for finely divided silica in plastics. However, it has been found that aqueous spray dried powder made in accordance with the method of Wolfgruber et al having triorganosiloxy units, chemically combined with tetrasiloxy units is incompatible with silicone fluids. Attempts therefore to utilize such silicone resin powder to produce heat curable silicone compositions, such as pressure sensitive adhesive (PSA) shown by Murakami et al, U.S. Pat. No. 4,774,297 have been unsuccessful.
In copending application Ser. No. 08/030,737, filed Mar. 12, 1993 is shown a procedure for making a silicone resin powder by spray drying an organic solvent dispersion of an organosiloxane hydrolyzate. The silicone resin powder is dispersible into silicone fluid and useful for making heat curable silicone compositions, such as PSAs. An organic solvent dispersion of an organosiloxane hydrolyzate, comprising triorganosiloxy units, or "M" units of the formula, EQU (R).sub.3 SiO.sub.0.5, (1)
chemically combined with tetrasiloxy units, or "Q" units of the formula, EQU SiO.sub.2, (2)
is spray dried to form a free flowing organosilicon resin powder having a primary particle size of 0.1-200 nm and an aggregate particle size of 10 nm to 200 microns. Although useful heat curable silicone compositions, such as PSAs can be formulated by spray drying the organic solvent dispersions of the organosiloxane hydrolyzates, the viscosites of the resulting PSAs, or component mixtures used in making the heat curable mixture sometimes exceed optimum processing requirements.
It would be desirable therefore to be able to make free-flowing organosilicon resin powder having a primary particle size in the range of 0.1-200 nm, and an aggregate particle size of 10 nm to 200 microns which would be useful in combining with silicone network ingredients, such as a vinyl substituted organopolysiloxane fluid, a silicon hydride siloxane fluid and platinum catalyst to form a PSA having improved processing characteristics.